This invention relates to a process for the production of pulp for making paper and other fibrous products from herbaceous plants. Specifically the invention relates to a process for improving the yield, strength and other properties of chemical pulp made from herbaceous plants, especially kenaf.
It is well known to use hardwood and softwood as sources of fiber in the manufacture of paper and other fibrous products. However, due to the decreasing supply and attendant increasing cost of conventional wood sources, more attention is being paid to alternative sources of fiber for paper and paperboard production.
Herbaceous plants do not develop persistent woody tissue, but instead die back at the end of each growing season. However, they can be an attractive alternative to conventional wood fiber sources because of their relatively high productivity in terms of tons of fiber per acre per year. Nevertheless, several problems exist with using herbaceous plants in conventional pulp making machines. For example, herbaceous plant tissue is generally much less dense than that of conventional wood sources and logistical problems exist in transporting and storing sufficient amounts of herbaceous plant material for use in producing commercial quantities of useable pulp. In addition the fibers are more delicate and not readily suited for use in conventional wood pulping processes.
The woody tissue of plants, comprised primarily of tracheids of vessels, is called xylem. Herbaceous plants may or may not contain xylem, however those containing xylem are believed to provide the most desirable raw material for modern papermaking pulp.
Dicotyledonous plants such as kenaf, hemp, jute etc., contain xylem and are therefore potentially useful in making pulp for fibrous products. Monocotyledonous plants such as grasses, corn, and bamboo do not contain xylem and are therefore less desirable.
Hemp is an excellent example of a dicotyledonous plant suitable for use in fibrous products pulp. However, hemp is a controlled substance in the United States and presents logistical problems in terms of its supply.
Kenaf is an annual dicotyledonous plant originally grown mainly in India and tropical Africa, but is now cultivated all over the world. It can grow to heights of twelve feet or more in a four to six month growing season, and can yield six to nine tons of whole plant fiber per acre. Kenaf""s relatively high agricultural yield and dicotyledonous structure make it an attractive candidate as a wood chip supplement. However, the bulkiness of herbaceous plants, such as kenaf can create transportation and storage problems not associated with conventional wood fiber.
Alternative methods of making chemical pulp from kenaf have been explored, but the requirement of special equipment and processes have limited the use of these methods on a commercial scale. For example, U.S. Pat. No. 3,620,910 to Lawrence describes methods for preparation of groundwood pulp from a fibrous plant such as kenaf by a semi-mechanical pulping process. In the process of the ""910 patent, kenaf is first screened to remove foreign materials and dirt, then soaked in water. After draining the excess water from the material, it is chopped and then processed in a screw conveyor to a solids content of fifty to sixty percent. The resulting material is then treated with an aqueous solution containing 10 parts each of sodium sulfite and sodium carbonate at atmospheric pressure and ambient temperature to uniformly wet the pieces. After this treatment, the material is fed to a disc refiner to produce groundwood pulp. This process can be effective on a limited scale, but it does not integrate well with conventional pulp and papermaking facilities used by the vast majority of the industry.
U.S. Pat. No. 4,106,979 to Ruffini et al describes methods for separating the stalk from the woody core of dicotyledonous plants such as kenaf. The process of the ""979 patent is carried out by defibrating kenaf in a mechanical pulper with steam treatment until the woody core is separated from the bark fibers. The stock is then divided into a long fiber bark fraction and a woody fraction. The woody fraction is used for refiner groundwood, chemimechanical or semichemical pulp production and the long fiber fraction is used as stock for production of bleached pulp. This process adds costly refinement steps and would also not be useful in most conventional chemical kraft pulping processes.
Accordingly, it is an object of the invention is to provide a process for the production of chemical pulp from herbaceous plants such as kenaf.
An additional object of the invention is to provide a process employing material from herbaceous plants for making pulp in a chemical pulp processor.
Still another object of the invention is to provide a process for the treatment of herbaceous plants such as kenaf, which facilitates economical transportation of the material to a pulp mill.
An additional object of the invention is to provide a process which facilitates the use of herbaceous plant material in combination with conventional wood chips in making chemical pulp for production of paper and other fibrous products.
A further object of the invention is to provide a process for treating herbaceous plants such as kenaf to improve their pulpability in a chemical pulping process.
Yet another object of the invention is to provide a process of the character described for the treatment of kenaf and other herbaceous plants in a chemical pulping process wherein the resulting pulp exhibits improved properties, especially in regard to the yield and strength of the pulp.
A still further object of the invention is to provide paper, paperboard and other fibrous products made in whole or in part of fibers from kenaf or other herbaceous plants which exhibit improved properties and/or which may be produced more economically than known products containing such fibers.
With respect to the above and other objects and advantages, the present invention provides a process for production of chemical pulp from herbaceous plants. In accordance with its more general aspects, the process comprises chopping or otherwise dividing at least a portion of a herbaceous plant into pieces, densifying the herbaceous plant pieces to provide densified agglomerates and chemically pulping the agglomerates to provide a chemical pulp thereof.
Due to its availability and other factors, kenaf is especially preferred for use in practicing the invention. Depending on the end use of the pulp, the entire plant may be used or the plant bark and core material may be separated and only the core material used.
The dividing of the plant into pieces is preferably carried out by chopping the plant (or a portion thereof) using equipment conventionally employed for hay, straw, and similar agricultural materials, and then densifying of the pieces is preferably carried out using a machine suitable for forming and compressing particulate solids into discreet agglomerates such as pellets, cubes, briquettes and the like. For convenience, the densified agglomerates will be referred to hereinafter as cubes, but it will be understood that, unless otherwise stated, that the densified agglomerates may have any shape or dimensions suitable to promote their pulpability in a digester.
The densified cubes are preferably chemically pulped using the well-known kraft cooking process, although other chemical or semi-chemical cooking or digesting processes may be used such as soda or sulfite cooking processes. The densified cubes may thus be cooked in an aqueous liquor containing reagents selected from the group consisting of sodium hydroxide, sodium carbonate, sulfuric acid, sodium sulfite, sodium sulfide, and quinones, said quinones being selected from the group consisting of anthraquinone, anthrone and phenothremiquine and alkyl, alkoxy and amino derivatives thereof. Furthermore, the densified cubes may be used alone or mixed with chemically pulpable material from other sources such as conventional softwood or hardwood chips and/or shredded recycled paper or paperboard material.
The process of the invention enables production of improved quality pulp for paper and paperboard manufacture formed entirely or in part of material from herbaceous plants. In particular, the process enables herbaceous plant material such as kenaf to be subjected to chemical pulping techniques with improved pulp yield and strength and without the need for significant or expensive additional process steps, and the chopping and densification steps may be carried out at or near the harvesting facility so that the bulk of material transported to the pulp mill is minimized. Also, while conventional pulp fiber sources are renewable, the time risk, and expense involved in replanting or reseeding and then maintaining timber stands through a multi-year growing cycle present distinct disadvantages compared with herbaceous plants which can be harvested on an annual or semi-annual basis with relatively low maintenance and processing expense.
As a result of these and other advantages, the invention enables increased and more economical use of readily available pulp fiber sources which are harvestable on a much more frequent basis than conventional pulp fiber sources.